To improve the fatigue strength of parts, such as automotive transmission gears, that require high fatigue strength, a shot-peening process is applied on a grain boundary oxidation layer formed on a metal surface of the part that has been treated by a gas carburization and quenching process. In the shot-peening process, as disclosed in Japanese Patent Early-Publication No. 9 [1997]-176792, the projected material (also called “shot”), whose particle size is 0.2 mm or less, is used to make the compressive residual stress as close to the surface as possible, and to reduce the surface roughness. It is intended to improve the contact pressure fatigue strength that is especially required for gearwheels.
With such a small size, the time needed to reach a predetermined coverage can be shortened, since there are many grams per unit weight. However, a certain time for treatment is needed, because of the problem of the stability of the projection device. For example, it is difficult to control the quantity of projected shot with a small particle size. As a result, the coverage of the shot-peening process on a metal surface that has been treated by a gas carburization and quenching process often becomes 500 to 1,000%, which is significantly larger than that of a usual shot-peening process.
Further, because the maximum shear stress expressed by Hertz' stress theory for shot with a small particle size is found at a shallow position, it is likely also found at any shallower position. Especially, a slack-quenching layer having tens of μm in thickness is formed on the surface of a carburizing article that uses RX gas, because of an oxidized grain boundary. (A, used herein, the slack-quenching layer, caused by the oxidized grain boundary, is often referred to as an “abnormal surface layer.”) Further, any added elements, e.g., Mn and Cr, which have been originally added to improve a quenching and hardening process, have a chemical attraction to oxygen. Thus they become an oxide in a crystal grain boundary area during the carburization and diffusion processes. As a result, because the hardness of an area where the densities of these elements are reduced is also reduced, the area can be readily ablated.
In these contexts, the above publication, No. 9-176792, explains the following.
The required surface roughness after the shot peening process is about 1 μm or less in respect of the surface roughness and abrasion resistance, to avoid a cracking because of fatigue due to the notch effect. Accordingly, to have a surface roughness be about 1 μm or less, the depth of an abnormal surface layer before the shot-peening process should be about 15 μm or less.
The depth of an abnormal surface layer of a steel part should be about 15 μm or less by a carburization and quenching process or a nitrocarburizing and quenching process, to improve the compressive residual stress after the shot-peening process. The steel parts are then treated by the shot-peening process, in which the shot diameter is 0.1 to 1.0 mm, and the speed of the projected shot particles is 60 to 120 m/s.
Namely, these explanations mean that the conventional shot-peening process aggressively ablates an abnormal surface layer.
By a microscopic observation of the state where similar metals come in contact with each other and slide against each other, it is seen that such a state is that of solid lubrication, where solids directly contact each other, even if lubricating oil exists. Typically, if similar metals are in the solid lubrication state, the adhesion force is increased, and thus the coefficient of friction is increased. As a result, the depth of wear can be increased. Therefore, it is preferable that any thin film lie in the contact interface of the solids, to decrease the adhesion force.